Question 203885

D =  ST                   ;                   S = D/T                ;                  T  =  D/S


Since the rocket’s speed upward was 5,000km/h for 10 km, then its time to make the journey upward was {{{10/5000}}}, or {{{1/500}}}hr. 


Since on its journey downward the rocket traveled at 200km/h for the same 10 km, then its time to make the journey downward was {{{10/200}}}, or {{{1/20}}}hr. 


Now, since the rocket traveled a total of 20 km, and its time over that period was {{{(1/500)+(1/20)}}} = {{{26/500}}} = {{{13/250}}}, its average speed for the entire journey is:   {{{20/(13/250)}}}


{{{20/(13/250)}}} = {{{(20/1)*(250/13)}}}, or {{{5000/13}}}  =  {{{highlight_green(384.6154)}}} km/h